Combine salt, pepper, garlic powder, onion powder, and oregano. Rub this spice mix all over the brisket, ensuring it’s well-coated.
Place the minced garlic and sliced onion in the bottom of the slow cooker. Lay the seasoned brisket on top.
In a bowl, whisk together the beef broth, soy sauce, and ketchup. Pour this mixture over the brisket.
Put the lid on top and set the heat to low. Cook for 8 to 10 hours, or until the brisket is soft and can be cut with a fork.
Once cooked, remove the brisket and let it rest. Meanwhile, if you prefer a thicker sauce, transfer the cooking liquid to a saucepan. Mix arrowroot powder with a bit of water to create a slurry, then whisk it into the cooking liquid. Heat until the sauce thickens to your liking.
Slice the brisket against the grain and serve with the thickened sauce.